As a new broadband access optical fiber technology covering the “last mile”, PON needs no active node and only a simple optical splitter needs to be installed. Thus PON has the advantage of saving optical cable resource, sharing bandwidth resource, saving equipment room investment, high security device, rapidity of setting up network, low cost of network building, and so on. As a primary type of present optical access network, PON has been widely applied.
PON takes the point-to-multipoint master-slave control structure. With reference to FIG. 1, an Optical Line Terminal (OLT) is a master node device, which is connected with a plurality of Optical Network Units (ONUs) through optical splitter, and the processes of the ONU registration, authorization of the data waiting to be transmitted, message interaction of Media Access Control (MAC) sub-layer and so on are performed at the OLT. The ONU is a slave node device, responding to the OLT finding message, reporting the sizes of data waiting to be transmitted, transmitting data of every port according to granting information, and a plurality of client terminals are accessed to the ONU to support multi-service access. With such frame structure as in optical access network, the problem of how to reasonably allocating bandwidth and effectively using network resource to meet service requirement is of great importance to PON MAC scheme.
In optical access network, there are two ways of allocating bandwidth: Static Bandwidth Allocation (SBA) and Dynamic Bandwidth Allocation (DBA). As to SBA, ONUs or its port is allocated with fixed bandwidth and unused time slot cannot be arbitrarily occupied. The disadvantage of SBA is low bandwidth utilization ratio and poor adaptive faculty to self-likelihood service with high burst ratio. DBA algorithm is a mechanism or algorithm for rapidly re-allocating bandwidth based on requirements of the current client service. As to DBA, the PON bandwidth utilization ratio can be increased by dynamically adjusting bandwidth, with reference to requirements of ONUs burst service.
At present, there are a lot of dynamic bandwidth allocation methods, For example, an Interleaved Polling with Adaptive Cycle Time (IPACT) dynamic bandwidth allocation method for PON; a DBA protocol special for APON which is put forward in G.983.4 standard by ITU-T (International Telecommunication Union-Telecommunication) Standardization Sector.
The main idea of IPACT is that polls the next ONU before the data sent by the former ONU arrive at OLT, thereby, to determine whether to authorize and how much to authorize, etc. and determine the start time of the data transmission of the next ONU according to the calculated arrival time of the last bit of the data of the current ONU.
As to the second scheme, a DBA protocol is put forward in ITU-T G.983.4 standard, service from ONU is divided into several types of T-CONT (Transmission Container). According to service priority, there are four types of T-CONT, in descending priority they are T-CONT1, T-CONT2, T-CONT3 and T-CONT4, while T-CONT5 is adaptive to all services in order to reduce number of transmission containers. Every type of T-CONT corresponds to a specific requirement of bandwidth allocation. There are four types of bandwidth allocation requirements: constant bandwidth, assured bandwidth, non-assured bandwidth and best-effort transmission bandwidth. And there are three strategies in the present DBA protocol: NSR (Non-Status-Reporting), SR (Status-Reporting) and mixing type.
In the first technology scheme mentioned above, the polling period adaptively varies with amount of data, resulting in data transmission jitter in the same service, so the requirement of low delay and delay jitter for real-time service is not be satisfied. In the second technology scheme, Round-Robin constant sequence polling is adopted, which is special for connection-oriented APON application with fixed length of PDU (Protocol Data Unit). Additionally, the bandwidth updating mechanism is special for T-CONT and bandwidth is allocated neither according to different ports nor different services.
In the above two schemes, since different allocating strategies for different services are not taken into account, they are applied in single service system in practical application, such as in system only providing browsing service or voice service. This kind of system cannot meet Quality of Service (QoS) requirement of more than one service type, neither allocate bandwidth according to client service ports. Moreover, when allocating bandwidth, granting messages of window allocation are basically generated in sequence of port reporting, without considering delay requirement of different ONU ports, and bandwidth utilization ratio is also low; in addition, there is no dynamic process if ONUs are offline.